Three years ago, when our daughter Rikki, then 17, shared a presentation on the dangers of kids exposing their personal information online, I was relieved to know our high schooler had a handle on this tricky terrain.

A few weeks later, on a whim, I logged on to my MySpace account, searched for Rikki, found her MySpace page, and then clicked on her friend Kristen's picture, which brought us to Kristen's page. There we could see that Rikki had posted, "Call me now!!!" Our home number followed — for Kristen and potentially the site's then 90 million other users to see, since Kristen had not made her page private. When we asked Rikki about it, she sheepishly admitted to having made an impulsive mistake and quickly asked Kristen to delete the post before the information could conceivably be used by stalkers, pranksters, or identity thieves.

Social-networking sites like MySpace and Facebook are incredibly popular with teens — recent surveys report that 71 percent of teens and 34 percent of 11- and 12-year-olds have a profile on a social-networking site (even though kids under 13 are not officially allowed on either site). Unfortunately, even the smartest kids — kids like Rikki, who know that you're not supposed to post private information online — make dumb mistakes on these sites, exposing themselves not just to predators and creeps, but also to bullying, identity theft, and other potentially negative consequences at school and, later on, in their work lives.

While it may be tempting to simply ban your child from using social-networking sites, a better strategy is to work with him to help him make smarter decisions online, says Monica Vila, cofounder of theonlinemom.com, a Web site devoted to educating parents about how to help kids use technology responsibly. (Teens whose parents have talked to them "a lot" about Internet safety are much less likely to take risks online, according to a Cox Communications survey.) Even if you are not on a social-networking site, or you have limited online skills, you can help your child avoid five of the most common flubs:

Mistake 1: Broadcasting Personal Info to the Entire Internet

Social-networking sites can provide a safe place for kids to exchange messages and connect with friends. But if your child's profile isn't set up so that only trusted friends can see the content, it's possible that everything he posts may be seen by anyone who's online. "About six months ago, I was contacted on Facebook by a guy in his 20s or 30s who had the same last name as me," says Courtney Marinak, 18, a recent high school grad from Jupiter, FL. "He said that we were related, and he was able to determine which one of his uncles was my father and even tell me what my sister's name was, all from information that was pieced together from my Facebook account. It turned out that he was legitimately related to me, but it was still scary having a stranger know so much about me."

The two giants of social networking, MySpace and Facebook, have improved their privacy practices in recent years, giving users much better tools for controlling who sees what. Those age 18 and under who set up profiles, for instance, are now given fairly strict privacy settings as a default, assuming they're honest about their ages. But many teens (not to mention their parents) don't know how to fine-tune their settings to keep themselves as safe as possible.

Arnold Bell, assistant chief of the FBI's Cyber Division Strategic Outreach and Initiatives Section, recommends sitting with your child when he sets up a Facebook or MySpace page and looking at the privacy settings, so you can make sure he chooses wisely. Experts advise using the strictest privacy settings (which are now generally automatic for kids 18 and under) from the start.

To do this on Facebook, go to the "Privacy Settings" link under "Settings" at the top right-hand corner of any page. Make sure all the menus under "Profile" and "Search" are set to "Only Friends" — meaning that only friends your child has approved can access his profile, photos, and other information — or possibly "Friends of Friends" if his goal is to connect with a broader group. On MySpace, find the privacy settings by clicking on the "My Account" link in the upper right-hand corner of any page. Then click "Privacy," and look for a heading that says "Profile Viewable By." Click on "My Friends Only." You can also customize other options — such as not allowing photos of your child to be shared or e-mailed by others — on the same page.

If your child already has an online profile, check to see if it's public by Googling his name or searching for him on Facebook or MySpace (you don't need to be a member to do so). Better yet, says Vila, do it together: "Say, 'I understand that these days anything that's posted online is very tough to keep private. Why don't we Google you to check if there's anything that you don't want out there?'" Also Google a friend or two of his, to compare and contrast. "You can say, 'This person has chosen to have this stuff about her online — what do you think about it?'" says Vila. "It's an opportunity for you to discuss what are the right values for your family."

Word to the wise: Also step in if your child is using Twitter, the micro-blogging service where users post short updates, known as tweets. Many kids don't realize these are completely public, akin to posting on a virtual global bulletin board. Help your child click on the "Protect My Updates" setting to control which members can follow what she posts. And if a stranger starts following her updates, she can restrict that person's access by using the "Block" tool.

Mistake 2: Sharing Passwords

Two years ago, 15-year old Julia Pullman* shared her MySpace password with friends. A few weeks later, she got into an argument with her friends, who retaliated by logging on to her MySpace page, spreading rumors about her being promiscuous, and generally savaging her reputation. "They also posted nasty things about other students, as if in her voice," her mother says. "It was just brutal." Julia was shunned by her classmates and has not yet socially recovered from the incident. Now she skips lunch at school to avoid eating alone, and has deleted her MySpace page.

It's common for teens to share passwords, says Vila — sometimes because of peer pressure, sometimes simply for convenience. But once that happens, the friend can log on as your child — whether to play a prank, like changing a name or profile description (common among teens), or for more malicious purposes.

Talk to your child about the importance of not sharing passwords with anyone but you, under any circumstances (you can use Julia's story as a cautionary tale). Then keep an eye on her pages for clues that someone else has been tinkering with them. (It should be a nonnegotiable rule that you yourself have an account and are part of your child's friend network, says Vila.) And let her know that if she breaks the rules, there will be consequences.

Word to the wise: Vila suggests punishing your child offline — not allowing her to attend a friend's party, for example — rather than banning her from a social-networking site, which might simply drive her to create a new account under a different screen name. Worth noting: Remind your child that it's critical to pick a password someone can't easily guess; that's a key step in protecting her online privacy.

Mistake 3: Befriending Strangers

It doesn't matter how strict your child's privacy settings are if she voluntarily adds people she doesn't know to her friends list. Teens often make a sport of accumulating friends on Facebook or MySpace, because it makes them feel popular. "You hear kids bragging, 'I have 785 friends,'" says Lisa Sohmer, director of college counseling at Garden School in Jackson Heights, NY. Scammers or predators can troll for such kids by sending out friend requests like spam, then using one child's network of friends to connect with others. Take the case of Katie Huntington, 17, of Oakland, CA, who describes herself as a cautious Facebook user. "I accepted one guy as a friend last year — we had lots of mutual friends," Katie says. "Then I checked his profile." He was an older man, and Katie saw many messages on his Facebook page from people asking, "Who are you?" Katie quickly deleted him from her account.

Most parents already worry that "stranger online" means "sexual predator." Of course, that's a serious and well-publicized concern--and one that the social-networking giants are increasingly addressing — but friending strangers also leaves your child vulnerable to unwanted offers of sales and services, not to mention identity theft. In the brief time Katie was friends with that unknown man, he could have downloaded her photos and recorded her birthday, and possibly other personal information, and used this info to open accounts in her name.

If your child is just getting started on a social-networking site, establish a rule that you must approve all friend requests, says Vila, "just like you would approve who she brings into your house." Also, friends should only be people your child knows personally. Then click around her friends list occasionally and, if someone looks out of place, ask about it.

Word to the wise: Remind even experienced social networkers to review their contacts regularly. Relationships shift dramatically at this age — last semester's BFF may be this semester's frenemy, and your child may want to bar certain "friends" from seeing her personal info.

Mistake 4: Baring Their Souls

Your average teen would never plaster the halls of her school with signs declaring whom she's got a huge crush on, how badly she flunked last week's algebra test, or what she really thinks about her uncle's drinking problem. Yet that's exactly what kids do when they open up and post about their personal lives online. "The meaning of 'friend' gets blurred when teens are on social networks," says Sohmer. "They share an intimacy of conversation that they would never have with those people in real life."

By broadcasting highly personal information, social-networking sites can magnify the usual teen and tween social dramas a hundredfold. "A cute boy in one of my classes started writing on my Facebook Wall, and I was so happy," says Emma Kincaid,* 14, a ninth grader from the New York City area. "The next day half of our grade knew that this boy and I were talking to each other because of the News Feed" — a Facebook feature where one's friends can get instantly updated on virtually one's every move on Facebook. Unfortunately for Emma, "half of our grade" included her crush's ex-girlfriend. "She screamed at me in front of the entire cafeteria," says Emma. "And for weeks afterward, she'd mock me and curse at me in the hallways."

Emphasize to your child that oversharing online can leave her open to bullying, ridicule, and social ostracism. "I tell my 15-year-old niece, anytime you put something on Facebook, it's like standing onstage in the school auditorium with a megaphone," says Vila; that's a message all kids should hear.

Mistake 5: Forgetting Their Futures

Teens frequently post photos from — or messages about — their wildest adventures on their social-networking pages, thinking that only their friends will see them. In fact, 54 percent of 18-year-olds on MySpace post about behavior such as sexual activity or substance use, according to a study published in the Archives of Pediatrics & Adolescent Medicine. But what many kids don't realize is that posting about these escapades can hurt them in the college-admissions race and future job searches.

"I've had several students who have been expelled from private high school or had admissions letters withdrawn because of drug- and alcohol-abuse images posted on Facebook," says Mark Truman, executive director of Omniac Education, a test-prep and college-consulting service in Arizona and New Mexico. Indeed, a recent study conducted at the University of Massachusetts Dartmouth found that 21 percent of colleges use social-networking sites to gather information about applicants.

Despite the risks, surveys show that 49 percent of teens are unconcerned that what they post online might negatively affect their futures. And while some may try to protect themselves by keeping their profiles private, the best protection is not to post anything incriminating in the first place. "The Web never forgets," says Scott Granneman, a technology expert and adjunct professor at Washington University in St. Louis. "You may post a picture and then delete it, but anybody can copy it when it's live and store it on a blog, Flickr [a photo sharing site], or their own Facebook page. You never know what can come back to haunt you later."

Help your kids avoid such problems by sharing some cautionary tales with them, and ask them if they think this could ever happen to them. But back that up by checking their pages — and their friends' pages — to see what turns up. And keep talking (and talking, and talking some more) about what's safe and not safe online, says Vila: "The more dialogue you have now, the less damage control you'll have to do later."

The Scoop on Sexting

As if social-networking snafus don't give parents enough to worry about, here's a new concern: sexting — in which tweens and teens use mobile devices to snap sexually charged photos of themselves and send the pictures to friends. A recent study found one in five teens had sent or posted a sexually explicit photo of himself or herself. For the kids, it may be just misguided hijinks, but it can have harsh consequences: Several teens have faced charges of disseminating child pornography due to photos they have taken of themselves or pictures of others they've forwarded. So be sure to talk with your kids about how sexting is one of the worst moves they could make.

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